Optical communications are often carried out using a plurality of signals, each signal occupying one of a plurality of wavelengths, with the signals carried by a single optical fiber. Each signal is routed to a desired port through a series of multiplexers and arrays of switches. A fiber carrying a combined signal comprising a number of wavelengths W may be supplied to a routing assembly capable of routing each wavelength to one of a number of outputs N. In some systems, each wavelength will be routed to a single output, with each output carrying a single wavelength. In such a case the number of wavelengths W is equal to the number of outputs N.
For example, a fiber may carry a combined signal comprising eight wavelengths, each of which is to be routed to one of eight ports, with each port carrying only one wavelength. At any time, any wavelength may be routed to any port. In the case of a signal comprising eight wavelengths, the combined signal is demultiplexed to form W signals of different wavelengths, for example using a demultiplexer having one input and W outputs, with the number W in the present exemplary case being eight. Each of the signals is routed to one of W switch arrays, with each switch array having one input and N outputs. In the case of an assembly having eight wavelengths and eight outputs, the number of switch arrays is eight and the number of outputs of each switch array is eight. Depending on the port to which the signal is to be routed, each signal is subject to be routed to any of the switch outputs of its switch array. The outputs of the switch arrays are supplied to N multiplexers, each having N inputs and one output, in this case eight multiplexers, each having eight inputs and one output. The first output of each array is supplied to a multiplexer whose output is in turn supplied to the first port, the second output of each array is supplied to a multiplexer whose output is in turn supplied to the second port, and so on. By suitable setting of each switch array, any signal may be supplied to any port.
Alternatively, it is possible for more than one wavelength to be carried by a single output port. Such a design allows for a routing system to be constructed for which the values of W and N are different. For example, a system might be constructed to route each of twelve wavelengths onto one of eight outputs. The system would include a one by twelve demultiplexer and twelve switch arrays having eight outputs each, feeding an array of eight multiplexers each having twelve inputs and one output.
One of two different alternative techniques is commonly used for signal routing. The first may be referred to as wavelength permutation routing, where only wavelength from the combined input signal is routed to a particular output port at the same time. The other technique may be called wavelength independent routing, and allows more than one signal to be routed to a single port at the same time.
A useful way to implement a switch array to be used for routing of optical signals is to construct the array as an array of silica waveguide based thermo optic switches. Such switches can route a single input to one of two outputs. Routing a signal to one output of such a switch requires a lower power dissipation, while routing a signal to the other output requires a higher power dissipation. Therefore, such switches can be thought of as having a low power position and a high power position, or a low power output and a high power output.
Prior art techniques for design of switch arrays seek to minimize the number of levels of an array, that is, the maximum number of switches through which the signal must pass, in order to minimize possible signal loss and the likelihood of errors. However, minimizing the number of levels of an array typically does not minimize the maximum and average power dissipation of the array, and wavelength independent routing in particular imposes power constraints on the arrays, requiring the worst case and average power dissipated by the array to fall within limits imposed by the desired design of the system in which the array is to be used.
The worst case power of an array is the power dissipated in the path requiring the highest power consumption, and the average power is the average power dissipation of the paths. The power dissipation of a path may suitably be defined as the number of switches in the high power position for the path under consideration. Increasing the power dissipation of an array leads to increased heat generation with the accompanying need to dissipate the heat. Minimizing the power dissipation of switch arrays employed by a routing system is particularly advantageous, because a device exhibiting lower power dissipation can be made more compact and less expensive.
If proper design techniques are used for a switch array, increasing the number of levels above the minimum necessary to support the desired number of outputs allows for significant decreases in the power dissipation of the array. In some cases, it may be necessary for the power dissipation of an array to fall within prescribed power constraints, with these constraints being more stringent if wavelength independent signal routing is to be used. There exists, therefore, a need for systems and techniques for developing switch arrays in which the switches are arranged so that the maximum and average power dissipation of the array, that is, the maximum and average number of switches in the high power position, is at the minimum that can be achieved given any limits on the number of stages of the array.